A variety of hydrocarbon applications involve the use of electrically powered equipment disposed within a hydrocarbon well for extended periods of time. For example, an electric submersible pump (ESP) may be positioned within a hydrocarbon well to promote the extraction of hydrocarbons from the well. In such circumstances it may be preferable to leave the pump in place and operating throughout the hydrocarbon production from the well. Thus, depending on the hydrocarbon reservoir itself and the parameters of the operation, the pump may be left operating and in place for up to about 5 years or longer.
Equipment such as the indicated ESP may include several components susceptible to damage upon exposure to the downhole conditions of the well. For example, the moisture content, chemical makeup, and pressure or temperature extremes of the downhole environment may tend to degrade certain components of the ESP over time. Components of the ESP susceptible to such exposure may include a power cable and motor parts such as motor windings or conductors. However, measures may be taken to help shield such components from the downhole environment. For example, in the case of the power cable, thick and robust, moisture resistant polymer layers may be extruded over an electrically conductive core. In this manner the core may remain substantially unaffected by downhole conditions so as to help ensure that the cable remains operation for an extended period. Alternatively, in the case of the motor and windings, they may be housed within an oil-filled and hermetically sealed casing isolated from the environment of the well.
Unfortunately, the oil filled casing noted above invariably fails to maintain complete isolation from the conditions in the surrounding downhole environment. For example, when left within the well for an extended period, moisture and chemical contaminants from the downhole environment are eventually able to seep through and penetrate the casing to some degree. Nevertheless, in the case of some parts of the motor, the fact that the casing remains predominantly oil-filled may be enough to avoid failure. For example, the moving parts of the motor may remain in the presence of sufficient lubrication to remain operational in spite of a degree of moisture and chemical contaminants. However, as described below, the direct exposure of the motor windings to the well contaminants, especially moisture, may be enough to render them ineffective, leading to malfunction of the entire ESP.
Unlike other parts of the motor, motor winding wires are not dependent upon the presence of sufficient oil concentration within the casing in order to remain operational. Rather, like the power cable, it is the substantial shielding of the motor winding wires from direct contact with downhole contaminants, especially moisture, which may be key to ensuring continued functionality of the wires. However, as indicated above, given enough time downhole, the casing is likely to be penetrated by such downhole contaminants leaving the wires directly exposed to contaminants.
In order to further shield the motor winding wires from direct exposure to downhole contaminants, polymer layers may be provided about the conductive core of the motor winding wires. Thus, in theory, the polymer layers may provide a degree of shielding to the motor winding wires similar to the power cable configuration noted above. Unfortunately, however, the dimensions and properties of the motor winding wires themselves render conventional polymer layering and shielding ineffective for prolonged protection of the wires from exposure to downhole contaminants. For example, a conventional motor winding wire may be magnetized wire core of no more than about 5 gauge copper wire, generally between about 16 and 50 gauge. Furthermore, the motor winding wire may be configured for relatively tight windings. As such, no more than between about 0.25 to 20 mil polymer layers may be effectively provided over the wires. In fact, for 30 gauge or so windings and smaller, as a matter of practicality it may be more effective to bypass extruding the polymer layer altogether and simply varnish the polymer over the wound wires to provide the shielding from downhole contaminants. Regardless, the polymer layer may be of limited thickness and effectiveness.
In addition to the limited thickness, the effectiveness of the polymer layer as a shield from downhole contaminants may be further limited by the particular types of polymers available for use with motor winding wires. That is, given the small dimension and the conductive nature of motor winding wire, materials disposed thereabout may be of an electrically insulating character to ensure proper wire operation. These materials may include polyimide, polyester, polyamide, poly-ether-ether-ketone and other conventional electrical insulators. Unfortunately, however, such insulators are prone to hydrolytic degradation or moisture absorption upon prolonged direct exposure to even a small degree of moisture and other downhole contaminants. As a result, the motor winding wire as well as the entire ESP or other equipment employing such winding wire is prone to fail, generally well in advance of about 5 years. In fact, smaller ESP motors positioned downhole for continued use often display a lifespan of no more than about 1 year. Furthermore, efforts to overcome polymer shielding limitations via over-wrapping or enamel layer configurations remain insufficient to prevent such hydrolytic degradation and moisture absorption.